1. Field of the Invention
This invention relates generally to decreasing audible noise caused by engagement between meshing gear teeth, as typically encountered in, for example, helicopters. More particularly, this invention relates to decreasing cabin noise in helicopters using an actuator to generate compensating torque pulsations that are transmitted to a meshing gear. The invention also relates to evenly distributing loads on a gear, such as a helical gear, spur gear, pinion gear or bull gear, thereby minimizing impulsive gear loading on each gear tooth.
2. Brief Description of the Art
Interior noise control (i.e., cabin noise control) is a primary concern in the operation of vehicles with rotor assemblies, such as helicopter. The noise is created by operation of the main rotor assembly during flight of the helicopter and results in the generation of xe2x80x9clow frequencyxe2x80x9d vibrations (i.e., frequencies less than approximately 300 Hz). Additionally, the operation of the main transmission of the helicopter generates xe2x80x9chigh frequencyxe2x80x9d vibrations (i.e., vibrations having a frequency between approximately 300 to 1000 Hz).
A major source of the high frequency vibrations is due to meshing between mutually engaging and rotating gear teeth. The gear teeth meshing causes unwanted multi-frequency noise in the vehicle cabin. This noise, aside from making the cabin environment unpleasant, makes understanding speech, and thus conversation, difficult. Gear tooth impact is a mechanical meshing of two inter-engaging gears that creates an undesired vibration, which is transmitted through the bearings to a gearbox housing. This undesired vibration is repetitive and periodic because there are typically multiple teeth on each gear. Since the gearbox is bolted to the airframe of a vehicle, the vibratory stresses are transmitted to the airframe, through the gearbox mounts, and produce acoustic noise, which is a function of the acoustic vibration, in the cabin of the vehicle. Although acoustic vibrations can be measured using a sensor, such as a strain gauge or other suitable vibration sensor, they are very difficult to correct.
U.S. Pat. No. 5,310,137, issued to Yoerkie, Jr. et al., entitled, xe2x80x9cHelicopter Active Noise Control Systemxe2x80x9d relates to an active noise control system (ANC) for a helicopter that is operative to effectively nullify one or more high frequency vibrations emanating from a main transmission gearbox at the gearbox/airframe interface. The ANC system includes modified transmission beams that are mechanically stiffened to function as rigid bodies with respect to high frequency vibrations.
U.S. Pat. No. 5,526,292, issued to Hodgson et al., entitled, xe2x80x9cBroadband Noise And Vibration Reductionxe2x80x9d relates to an active noise and vibration cancellation system with broadband control capability. A broadband disturbance signal detector positioned within a closed compartment, such as an aircraft cabin or vehicle passenger compartment, provides a signal representative of the frequency spectrum and corresponding relative magnitude of a broadband signal emanating from a vibrational energy source to a controller. The controller receives the broadband disturbance signal and error signals from error sensors that enhance the cancellation capability of the control signals produced by actuators that are positioned within the compartment.
U.S. Pat. No. 5,853,144, issued to Vincent, entitled, xe2x80x9cHelicopter And Method For Reducing Vibration Of A Helicopter Fuselagexe2x80x9d relates to a helicopter structure with a fuselage, rotating system parts capable of relative motion at an exciting frequency and a plurality of actuators. A plurality of sensors are attached to the rotating system at selected locations. Processing means are adapted to process signals from the sensors and provide output signals for controlling the phase and magnitude of applied forces generated by the actuators to reduce the overall level of vibration in the fuselage.
U.S. Pat. No. 5,485,761, issued to Rouverol, entitled, xe2x80x9cArticulated Differential Crowningxe2x80x9d relates to a system of modifications for power train gearing. The gear teeth are modified in an attempt to eliminate transmission error at all loads so that the dynamic increment of load is also eliminated at all loads.
However, none of the above-cited patents adequately reduce noise in a vehicle cabin due to impulsive gear clash loads such as encountered in helicopter operation.
One conventional approach to reduce acoustic vibration produced in a vehicle gearbox is to modify the gear tooth design in an attempt to reduce impact, e.g., utilize a herring bone gear, which is a gear with a more complex gear tooth design. Unfortunately, this gear design has been found not to sufficiently decrease acoustic noise. Also, straight spur gear teeth have been used, although less expensive to manufacture, these gears tend to separate under force, which is unsatisfactory.
A second conventional approach to reduce the acoustic vibration produced in a vehicle gearbox, and thus acoustic noise, is to isolate the gearbox from the vehicle airframe. One such isolation technique utilizes vibration isolators, for example, rubber pads disposed between the gearbox and the airframe to isolate the gear clash loads produced in the gearbox. Unfortunately, static loads on the vehicle cause deflection of the gearbox during flight deforming the rubber pads between the gearbox and the airframe thus causing the airframe and gearbox to experience movement relative to each other. This movement can bind control rods, which are mounted between the gearbox and the airframe, causing the control rods to deform or otherwise become damaged or to inadvertently apply anomalous controls to the main rotor. Additionally, when the gearbox experiences movement, the alignment of the drive shaft may be altered such that the drive shaft experiences undesired movement as a result of the vibration. This undesired movement can cause the drive shaft to bind, deform, bend or otherwise become damaged.
Thus, padding materials that are suitable to absorb the vibratory loads between the gearbox and the airframe can result in undesired movement that can bind or damage control rods and/or drive shaft(s).
A third conventional approach to reduce acoustic vibration produced in a vehicle gearbox is to mount special active noise cancellation (ANC) actuators between the gearbox and the airframe at each possible load path. This method is expensive to implement because there are typically a large number of load paths. For example, in the S-76(trademark) helicopter (S-76 is a trademark of Sikorsky Aircraft, a division of United Technologies Corporation) the number of load paths is 24; (4 gearbox mounting feetxc3x976 motions per foot=24), which requires up to 24 ANC actuators. Moreover, the actuators can usually only effectively reduce one frequency of a multi-frequency vibration thus allowing other frequencies of the vibration to pass into the cabin unmitigated. Each actuator has a directed cost, which is the actual cost of the actuator, as well as indirect costs, which are due to the additional mass of each actuator and the associated additional power demands on the vehicle. Thus, the use of more than the minimum number of actuators is undesirable.
A fourth conventional approach to reduce vibrations produced by a vehicle gearbox is to place a polymer material, such as an elasto-polymer or foam rubber, in the gearbox, on the gears or gearbox housing surface, to absorb vibration. However, the gearbox environment also includes fluids necessary for lubrication, such as transmission fluid, that degrade such polymer materials rapidly, thereby diminishing the effectiveness of such materials to absorb vibration. Thus, the environment of the gearbox interior makes this approach unsatisfactory.
A fifth conventional approach is an arrangement to reduce acoustic noise from gearboxes by applying transverse loads to the gear mounting bearings usually using piezoelectrical actuators. This method has several shortcomings. Most importantly, the meshing gears need to be firmly positioned relative to each other so that proper meshing occurs. This requires the mounting bearings and surrounding gearbox housing backup structure to be very stiff. This being the case, any actuator that attempts to move the mounting bearings or structure must produce prohibitively high loads to effectively reduce the gear tooth loads.
Thus, what is needed is a system to effectively reduce the acoustic vibration created by gearbox noise without compromising the structural relationship between the gearbox and the airframe or adding undesired mass and/or expense to the vehicle or by introducing large loads into the gearbox structure.
The present invention relates to an apparatus and method to reduce acoustic vibration in a vehicle cabin or cockpit due to impulsive gear clash loads produced by gear teeth.
One embodiment of the instant invention is directed to an apparatus for reducing noise in a vehicle. The apparatus includes one or more sensors that are mounted on the vehicle and used to sense noise. A first gear, having a plurality of teeth and a second gear, also having a plurality of teeth, are disposed such that upon rotation of the first and second gears, certain ones of the second gear teeth mesh with certain ones of the first gear teeth. An actuator, which is coupled to the first gear, drives the first gear and generates torque pulses that are transmitted to the first gear. A controller is coupled to the actuator and controls the torque pulses that the actuator transmits to the first gear. The torque pulses generated by the actuator are electrically phased and magnitude adjusted so that the first gear teeth vibrate at a predetermined frequency thereby reducing vibrations caused by meshing of the first gear teeth and the second gear teeth.
Another embodiment of the instant invention is directed to a method for reducing vehicle vibrations caused by gearbox vibration. A first gear, which has a plurality of teeth, is rotated and a second gear, which also has a plurality of teeth, is rotated such that the first gear teeth and second gear teeth interact to produce impulse loads that are transmitted through support bearings to a gearbox housing. Vehicle vibrations, generated by the impulsive noise, are sensed and a command signal is generated, as a function of the sensed vehicle vibrations. The command signals are transmitted to an actuator. Pulsating torques are generated by the actuator in response to the command signals and transmitted to the first gear. The pulsating torques are electrically phased and magnitude adjusted to cause the first gear teeth to vibrate at a predetermined frequency, thereby reducing vibrations caused by meshing of the first gear teeth and second meshing. The steps of this method are suitably stored on a computer-readable medium.
Yet another embodiment is directed to an apparatus for reducing noise in a vehicle. This apparatus includes sensing means, mounted on the vehicle, for sensing noise. A first gear, having a plurality of teeth and a second gear, having a plurality of teeth, are disposed such that upon rotation of the first and second gears, certain ones of the second gear teeth mesh with certain ones of the first gear teeth. Actuator means are coupled to the first gear, for driving the first gear and for generating torque pulses that are transmitted to the first gear. Control means are coupled to the actuator means, for controlling the torque pulses that are transmitted to the first gear. The torque pulses generated by the actuator means are electrically phased and magnitude adjusted so that the first gear teeth vibrate at a predetermined frequency to minimize vibrations caused by the first gear teeth and the second gear teeth meshing.
Yet another embodiment of the instant invention is directed to a method and apparatus for generating pulsating torques to minimize impulsive loading on gear teeth by reducing variance between peek and non peak loads.